High-temperature non-oxidizing water vapor absorbent

ABSTRACT

A new absorbent for water vapor is disclosed which is non-oxidizing and is suitable for use as the absorption working pair in combination with H 2  O in absorption heat pumps, and also as a liquid drying agent. The absorbent is comprised of a mixture of at least two alkali metal thiocyanates, preferably KCNS and NaCNS. The preferred weight ratio is between 3 and 4, which remains liquid at all water vapor pressures above 1/16 ATA (curve &#34;30 Na&#34; on the Figure). Further advantages are obtainable by incorporating at least one of lithium cations, cesium cations, and hydroxide anions in the absorbent mixture.

DESCRIPTION

1. Technical Field

This invention relates to liquid absorbent compositions which absorbuseful quantities of water vapor at high boiling-point elevations, andwhich are useful in absorption cycle devices such as absorption heatpumps, solution compressors, and gas dryers. The disclosed solution andthe cycles which employ it provide the combined advantages of highersolubility limit at low temperatures, high-temperature stability,non-oxidizing character, and compatibility with common constructionmaterials such as austenitic stainless steel.

2. Background Art

There has long been an identified need for an absorbent for water vaporwhich overcomes the various problems of the currently used absorbents,most particularly LiBr. Aqueous LiBr solutions suffer two primarylimitations when used as a water vapor absorbent. The solubility field(i.e., the maximum boiling point elevation) is quite limited at lowertemperatures, and the corrosivity is quite severe at higher temperatures(e.g., above 160° C.). Also it is not compatible with austeniticstainless steel at high temperatures.

Many additives to LiBr have been tested for possible amelioration of theabove limitations. These include CaCl₂ (U.S. Pat. No. 2,143,008), CsBr(U.S. Pat. No. 3,004,919), LiSCN (U.S. Pat. No. 3,541,013), LiI (U.S.Pat. No. 3,524,815), ZnBr.sub. 2 (U.S. Pat. No. 3,478,530) and ethyleneglycol. Much of the earlier work is summarized in the 1978 technicalreport, "Candidate Chemical Systems for Air Cooled, Solar Powered,Absorption Air Conditioner Design," by W.J. Biermann (SAN-1587-2, 1978),published by the U.S. Department of Energy, Washington, D.C.

In overall summary, although some of the additives do improve oneparameter, it is invariably at the expense of a degradation in someother important parameter (viscosity, thermal stability, etc.) such thatno net benefit is realized. Hence the only additives routinely used inLiBr solutions are trace amounts of corrosion inhibitor (e.g., LiOH plugchromate or molybdate) and trace amounts of mass transfer enhancer(e.g., octyl alcohol), and the historic limitations of LiBr solutionspersist.

No other single salt is known to evidence as wide as low temperaturesolubility field as LiBr. Recently, however, certain mixtures ofnon-halide salts have been discovered to exhibit useful new capabilitiesas water vapor absorbents. Both the binary NaOH-KOH and the ternaryNaOH-KOH-CsOH have been found to greatly extend the low temperaturesolubility limit (U.S. Pat. No. 4,614,605). However, those mixtures havethe disadvantages of being caustic, moderately corrosive, and reactivewith atmospheric CO₂.

A second advantageous mixture is the family of alkali nitrates (U.S.Pats. Nos. 4,454,724, 4,563,295, and 4,652,279). These mixtures arethermally stable to well above 260° C., and virtually non-corrosive tomany materials of construction including mild steel. However, they havethe drawbacks of having a severly limited low temperature solubilityfield, and also their potentially oxidizing nature. This causes concernwhen the absorbent is in heat exchange relationship with a fuel species,as a leak in the heat exchanger would allow the fuel and oxidizer to mixwhich could result in uncontrolled combustion.

Another problem with existing non-volatile liquid-phase waterabsorbents, including all of the halide, hydroxide, and nitrate mixturesreferenced above, is that their anhydrous melting point is too high,typically well above 100° C. When it is desired to dry a gas such asammonia synthesis gas or hydrocarbons to a "bone-dry" condition, it isnecessary to contact the gas with a virtually anhydrous absorbent. Thelower the contact temperature, the more H₂ O the absorbent can absorb,and the lower the regeneration temperature, along with other advantages.Hence it would also be desirable to have a non-oxidizing liquidabsorbent of water vapor with an anhydrous melting point of below 100°C.

What is needed, and one objective of the presently disclosed invention,is a new water vapor absorbent which avoids the limitations enumeratedabove of the halide-containing systems, the hydroxide mixtures, and thenitrate mixtures. More particularly an absorbent is desired which isnon-oxidizing, and which has at least as extensive a solubility field asthe nitrates. It would be preferable if it could be used at even lowerwater vapor pressures than the nitrates, and also at comparably hightemperatures as the nitrates.

SUMMARY OF INVENTION

The above and other desirable objectives are obtained by providing asthe water vapor absorbent solution a mixture comprised of potassium andsodium thiocyanates.

The liquid absorbent is normally utilized as an aqueous solutioncomprised of between about 2 and 60 weight percent H₂ O. However, due tothe low anhydrous melting point of the mixture, the absorbent can beregenerated to virtually anhydrous condition, thereby making possiblethe drying of gases to a "bone-dry" condition. When "bone-dry" drying isdesired, it is preferred to use a low melting point absorbent, andhence, the absorbent mixture would preferably also be comprised of LiSCNand/or CsSCN, since they further reduce the anhydrous melting pointbelow the 122° C. eutectic of the potassium-sodium-thiocyanate mixture.

Another use of the new absorbent is in an absorption cycle apparatus, asdescribed in the prior art references, wherein the reversible absorptionand desorption of water vapor is used to effect a transfer of heat fromlower to higher temperature. In one scenario, characteristic ofindustrial use, low grade heat at a temperature in the approximate rangeof 30° C. to 150° C. is upgraded to a higher temperature, preferablyfrom 30° to 90° C. higher than the input temperature. This may be viaeither forward cycle absorption heat pumps ("heat amplifiers"), reversecycle AHPs ("heat transformer" or "temperature amplifier"), or any knowntypes of advanced cycles--multieffect or staged, generator to absorberheat exchanger, etc. Examples of advanced cycles are presented in the1984 technical article "Analyses of Advanced Residential Absorption HeatPump Cycles" by B.A. Phillips, appearing in the U.S. Department ofEnergy publication CONF-841231. When the absorbent mixture is limited topotassium and sodium thiocyanates, the thermal stability is excellent,extending to above 200° C. If the preferred proportions of about 70weight percent KSCN and 30 weight percent NaSCN (anhydrous basis) isused, then water vapor pressures as low as 0.1 atmospheres can bepresent in either the generator or the absorber without possibility ofsolution crystallization.

By adding either LiSCN or CsSCN to the mixture, even lower water vaporpressures are possible, e.g., 0.01 (ATA) (corresponding to an evaporatoror condenser at about 7° C.). However, there are certain offsettingdisadvantages--the LiSCN is not thermally stable at high temperatures(above about 150° C.), and the CsSCN is quite expensive ($60/kg) andincreases the required solution circulation rate. Hence, the LISCNand/or CsSCN additives would only be expected to be used when either lowwater vapor pressures are present or when a low anhydrous melting pointis desired. Typical examples of heat pumping applications involving lowwater vapor pressures are space conditioning (heating or airconditioning) and hot water heating. The virtually unlimited solubilityfield of mixtures containing LiSCN and/or CsSCN make possible air-cooledair conditioning and also high-temperature high-COP generator absorberheat exchanger (GAX) cycles (CsSCN only).

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates the vapor-liquid-solid equilibrium (VLE)characteristics of aqueous solutions of various pure alkali thiocyanatesand their mixtures, on coordinates of RT1nP (kcal/mole) versus T(°C.),with constant pressure contours indicated.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the FIGURE, a series of experiments were conducted whichentailed boiling steam out of various alkali metal thiocyanate saltsolutions (pures and mixtures) at atmospheric pressure, plus cooling thesalts at constant concentration from various boiling points untilcrystallization was observed. With that data, plus an estimate of theslope of constant concentration VLE lines, it was possible to constructapproximate crystallization curves. Several curves from mixtures ofinterest are drawn on the figure. The slope of constant concentrationVLE, which is approximately the ΔS° of evaporation (change in standardstate entropy between liquid and vapor phase), can be estimated fromliterature values for pure salts or by analogy to similar salts havingsimilar heats of solution. Trouton's Rule reveals that most liquids havea slope of approximately 22 cal/mole-K. For most mixtures an anhydrousmelting point was also obtained, which provides a vertical asymptote toeach curve.

Because of the above estimation technique, the crystallization curvesare somewhat approximate, but nonetheless adequate to provide a goodoverall indication of the range of characteristics possible with thenewly defined salt mixtures as water vapor absorbents.

The water dissolving properties of the alkali metal thiocyanates havebeen found to be qualitatively similar to those of the alkali metalnitrates. The lithium cation provides the best boiling point elevationsand generally best solubility limits, but is by far the least thermallystable. The thiocyanates of the other alkali metal cations are highlystable, but much less soluble in H₂ O and with substantially reducedboiling point elevations (at a given concentration).

As with the nitrates, none of the pure thiocyanates is an acceptablewater vapor absorbent, for the above reasons. However, just about allmixtures of two or more alkali metal thiocyanates which contain no morethan about 85 weight percent (w/o) of any single thiocyanate species(anhydrous basis) have been found to be potentially useful as a watervapor absorbent. Even more surprising, a mixture which is limited toonly potassium and sodium thiocyanate has been found to be as good as orbetter than the best possible ternary mixture of LiNo₃, NaNO₃, and KNO₃as a water vapor absorbent. Most importantly, the binary thiocyanatemixture is non-oxidizing, and hence can safely be used with fuel speciessuch as hydrocarbons.

FIG. 1 illustrates the solubility fields of several alkali metalthiocyanates and their mixtures. Pure NaSCN and KSCN have very narrowsolubility fields until excessively high water vapor pressures arereached (0.5 ATA for KSCN and much higher for NaSCN). However, over therange of 20 to 35 w/o NaSCN in KSCN (anhydrous basis) much lower watervapor pressures are possible without crystallization: 1/8 ATA at 25 w/oNaSCN, 1/16 ATA at 30 w/o NaSCN, and 1/4 ATA at 36 w/o NaSCN. Note thatthe minimum possible water vapor pressure to prevent freezeup for theK-Na binary is found in that range of concentration, at roughly 30 w/oNaCNS.

If even lower mimimum water vapor pressures or lower anhydrous meltingpoints are required, they can be achieved by addition of CsSCN, LiSCN,or both. The figure shows an example effect of each type of addition,one curve for 16 w/o LiSCN, 58 w/o KSCN, and 26 w/o NaSCN. and the otherfor ternary equimolar K-Na-Cs thiocyanate. Other mixtures with even moreLiSCN evidence even lower minimum water vapor pressures, on the order of0.01 ATA or less. Also, certain quaternary mixtures have lower anhydrousmelting points than the 88° C. of the equimolar ternary.

It is within the scope of this invention to include trace amounts ofhydroxide anion in the thiocyanate mixture for pH control in order toreduce corrosion. It is also within the scope to incorporate verysubstantial amounts of hydroxide anion (e.g., up to 50%), therebyfurther improving the low temperature solubility field. This isparticularly advantageous with only potassium and sodium cations, sincethe increased solubility field is obtained without either the lowthermal stability of LiSCN or the high cost of CsSCN. Other additivesmay also be present in the thiocyanate mixture, such as mass transferenhancers, corrosion inhibitors, or freezing point depressants. However,nitrates and other strong oxidizers are not recommended for safetyreasons.

The essential components of the absorption cycle apparatus in which thisnew absorbent is used to absorb water vapor are a generator, anabsorber, and an absorbent solution circulation pump. These componentsform what is commonly referred to as a "solution compressor". Normallythere would also be present either a solution heat exchanger or agenerator-to-absorber heat exchanger (GAX cycle). Closed absorptioncycles also incorporate an evaporator and condenser in addition to thesolution compressor. However, the absorption heat pump may also be opencycle, in which case either or both of the evaporator and condenser canbe eliminated. When both are deleted, the solution compressor is inessence a thermally-powered steam compressor. In "forward" cycles, thegenerator water vapor pressure is higher than that of the absorber,whereas the reverse cycles the pressures are reversed.

I claim:
 1. A process for transferring heat from low to high temperaturecomprising evaporating water at said low temperature, absorbing theresulting water vapor into an aqueous solution at said high temperature,and providing as said aqueous solution a composition of matter comprisedof at least 2 weight percent H₂ O, KSCN and NaSCN.
 2. Process accordingto claim 1 wherein said high temperature is above 170° C.
 3. Processaccording to claim 1 further comprising regenerating said aqueoussolution for additional absorption by desorbing water vapor from it at apressure which is different by at least a factor of 1.3 from thepressure of said absorbing step.
 4. The process of claim 1 wherein thecomposition comprises between about 2 and 70 weight percent water. 5.The process of claim 1 wherein the weight ratio of KSCN to NaSCN isbetween about 4 and
 1. 6. The process of claim 5 wherein the weightratio of KSCN to NaSCN is about 2.5.
 7. The process of claim 1 whereinthe composition further comprises CsSCN or LiSCN or a mixture thereof.8. The process of claim 1 wherein the composition consists essentiallyof 40 to 80 weight percent KSCN and at least 10 weight percent NaSCN. 9.The process of claim 1 wherein the composition consists essentially of30 to 70 weight percent KSCN, 10 to 50 weight percent NaSCN and at least5 weight percent CsSCN.
 10. The process of claim 1 wherein thecomposition further comprises hydroxide ions.